Through Aaron I found out about Yahoo! opening up their user authentication to third parties. The ease of having just a single log in for various sites worldwide is very userfriendly. I applaud the idea. However, not the implementation.
As Aaron also notes, it can be confusing for users to be able to sign in with a Yahoo! account, but then if your site needs more information from the user, having to go through some kind of "registration" process. Single sign-on would be much better if it can be done in the background, using webservices. I can imagine though that Yahoo! would not like this too much, because that would mean the username and password would pass through the third-party application. Not good either.
The effort from Yahoo! is good, but again I don't like the implementation. I don't think I'll support the Yahoo! login any time soon on any of my websites.